The present invention relates to a process of producing chlorine dioxide. The process comprises reducing chlorate in an acidic reaction medium, precipitating and removing alkali metal sulfate from the reaction medium which is acidified electrochemically and recirculated to a chlorine dioxide generator.
Chlorine dioxide used in aqueous solution is of considerable commercial interest, mainly in pulp bleaching, but also in water purification, fat bleaching, removal of phenols from industrial wastes etc. It is therefore desirable to provide processes in which chlorine dioxide can be efficiently produced. Since chlorine dioxide is unstable the production normally occurs on-site, for example at pulp- and paper mills.
There are numerous different processes for chlorine dioxide production. Most processes in commercial use involve reaction of alkali metal chlorate, normally sodium chlorate, in an acidic medium with a reducing agent such as hydrogen peroxide, methanol, chloride ions or sulfur dioxide. The acidity is generally provided by sulfuric acid. Since alkali metal is introduced with the chlorate it also has to be removed from the system. This can be achieved in highly efficient processes run at subatmospheric pressure and involving formation of solid sodium sulfate as a by-product which traditionally has been used in the pulp- and paper mills. Such processes are described in, for example, U.S. Pat. Nos. 4,770,868, 5,091,166 and 5,091,167.
However, modern technology has reduced the need for sodium sulfate in the pulp industry and it is hard to find use for all sodium sulfate that is obtained. Thus, although some sodium sulfate still is needed in the pulp mills, a great deal of the formerly valuable by-product is now regarded as a useless waste material.
It has been disclosed that sodium sulfate obtained in chlorine dioxide production can be converted to sulfuric acid and sodium hydroxide in electrochemical cells.
U.S. Pat. No. 4,129,484 discloses a process of producing chlorine dioxide in which waste solution containing sulfuric acid and/or sodium sulfate is withdrawn from the reactor and acidified electrochemically.
U.S. Pat. No. 5,198,080 and 5,122,240 disclose a process of producing chlorine dioxide involving crystallisation and withdrawal of solid sodium sesquisulfate. At least some of the sesquisulfate is dissolved in water and acidified electro-chemically. The process is complicated since it requires that the sodium sesquisulfate first is precipitated and separated from the reaction medium and then it must be dissolved again before the electrochemical treatment.
International patent specification WO 93/25470 discloses a process of producing chlorine dioxide in which at least some of the acidity in the reaction medium is provided by chloric acid, thus introducing less alkali metal in the chlorine dioxide reactor.
The main object of the present invention is to provide a process of producing chlorine dioxide in which the amount of solid alkali metal sulfate obtained as a by-product easily can be controlled with regard to the need for alkali metal sulfate in each specific case. It is another object of the invention to provide a process with a reduced number of necessary unit operations.